Q7
(a) Distinguish the "T" (tense) and 'R' (relax) conformations of Hemoglobin on reversible binding of oxygen (O₂). 15 marks (b) What is the basic difference between Cytochrome 'b' and Cytochrome 'c' ? Explain the role of Cytochrome 'c' oxidase. 5 marks (c) Draw the possible stereoisomers of the following complex and explain their optical activity. 10 marks (d) (i) Consider a second-order reaction : A + B → P where the initial concentration of A is 'a' mol dm⁻³ and that of B is 'b' mol dm⁻³. After time 't', x mol dm⁻³ of A and x mol dm⁻³ of B react to form x mol dm⁻³ of the product, P. Show that the second-order rate constant for this reaction will be given by k₂ = 1/((a-b)t) ln [b(a-x)/a(b-x)] with the assumption that a > b. What will be the unit of k₂ ? (Consider time in seconds) (ii) Determine the units of the rate constants for zeroth-order and 5/2 order reactions. Assume that concentrations are expressed in mol dm⁻³ and time in seconds. 10 marks (e) Complete the following reaction and explain the mechanism with the help of pi-bonding theory. 10 marks
हिंदी में प्रश्न पढ़ें
(a) ऑक्सीजन (O₂) के हीमोग्लोबिन के साथ उत्क्रमणीय बंधन पर 'T' (टेंस) और 'R' (रिलैक्स) संरूपणों में अंतर स्पष्ट कीजिए । 15 अंक (b) साइटोक्रोम 'बी' और साइटोक्रोम 'सी' में मूल अंतर क्या है ? साइटोक्रोम 'सी' ऑक्सीडेज की भूमिका की व्याख्या कीजिए । 5 अंक (c) निम्नलिखित संकुल के संभव विसम समावयवों को बनाइए और उनकी द्रुवण घूर्णकता की व्याख्या कीजिए। 10 अंक (d) (i) द्वितीय कोटि अभिक्रिया पर विचार कीजिए : A + B → P जहाँ A का आरंभिक सांद्रण 'a' mol dm⁻³ है और B का 'b' mol dm⁻³ है। 't' समय के बाद, A के x mol dm⁻³ और B के x mol dm⁻³ आपस में अभिक्रिया करते हैं और x mol dm⁻³ का उत्पाद, P बनाते हैं। a > b मानते हुए, प्रदर्शित कीजिए कि इस अभिक्रिया की द्वितीय कोटि का वेग स्थिरांक k₂ = 1/(a-b)t ln [b(a-x)/a(b-x)] होगा। k₂ की इकाई क्या होगी? (समय सेकंड में मान लीजिए) (ii) शून्य कोटि और 5/2 कोटि की अभिक्रियाओं के वेग स्थिरांक की इकाई ज्ञात कीजिए। मान लीजिए कि सांद्रण को mol dm⁻³ और समय को सेकंड में व्यक्त किया जाता है। 10 अंक (e) निम्नलिखित अभिक्रिया को पूरा कीजिए और इसकी क्रियाविधि का pi-आबंधन सिद्धांत की सहायता से व्याख्या कीजिए। 10 अंक
Directive word: Distinguish
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How this answer will be evaluated
Approach
Begin by distinguishing T and R states of hemoglobin with structural details (part a, ~30% time). Then contrast cytochromes b and c, explaining oxidase function (part b, ~10%). Draw and label stereoisomers with optical activity analysis (part c, ~20%). Derive the second-order rate equation with proper integration steps, state k₂ units, then determine units for zeroth and 5/2 order reactions (part d, ~20%). Complete the reaction with pi-bonding mechanism explanation (part e, ~20%). Conclude with integrated biological significance.
Key points expected
- T (tense) vs R (relaxed) hemoglobin conformations: T-state has constrained Fe²⁺ out of porphyrin plane with low O₂ affinity; R-state has Fe²⁺ in-plane with high O₂ affinity; cooperative binding and Hill coefficient ~2.8-3.0
- Cytochrome b (membrane-bound, b-type heme with two iron protoporphyrin IX, no covalent attachment) vs cytochrome c (peripheral membrane, c-type heme with covalent cysteine thioether bonds); cytochrome c oxidase as Complex IV with Cu_A, Cu_B, heme a, heme a₃, proton pumping, 4e⁻ reduction of O₂ to H₂O
- Stereoisomers of octahedral complex [M(AA)₂B₂] type showing cis and trans forms; cis-[M(AA)₂B₂] as optically active (chiral, non-superimposable mirror images) and trans as optically inactive (achiral with plane of symmetry)
- Derivation of second-order rate law: dx/dt = k₂(a-x)(b-x), integration using partial fractions to obtain k₂ = 1/[(a-b)t] × ln[b(a-x)/a(b-x)]; unit of k₂ as dm³ mol⁻¹ s⁻¹
- Units determination: zeroth-order as mol dm⁻³ s⁻¹; 5/2 order as dm^(15/2) mol^(-5/2) s⁻¹ or dm^7.5 mol^-2.5 s⁻¹
- Reaction completion with pi-bonding mechanism: nucleophilic attack on metal-carbonyl or alkene complex with back-bonding explanation using molecular orbital theory
Evaluation rubric
| Dimension | Weight | Max marks | Excellent | Average | Poor |
|---|---|---|---|---|---|
| Concept correctness | 25% | 12.5 | Precisely distinguishes T/R hemoglobin states with structural details (Fe position, salt bridges, Bohr effect); accurately contrasts cytochrome b vs c heme types and membrane localization; correctly identifies chiral vs achiral stereoisomers; flawless derivation logic for rate law | Basic distinction of T/R states without structural specifics; generic cytochrome differences without heme chemistry; partial stereoisomer identification with confused optical activity; minor errors in integration approach | Confuses T/R states or omits Fe positioning; fails to distinguish cytochrome types or misidentifies oxidase function; incorrect stereoisomer assignment; fundamental errors in rate law derivation |
| Mechanism / equation | 25% | 12.5 | Complete derivation with partial fraction decomposition shown; explicit pi-back-bonding explanation with orbital diagrams for CO/alkene complexes; clear electron flow in cytochrome c oxidase mechanism | Final rate equation stated without full derivation steps; basic pi-bonding mentioned without orbital specificity; oxidase role described without mechanistic detail | Incorrect rate equation or missing derivation; no pi-bonding explanation or confused with sigma bonding; absent or wrong oxidase mechanism |
| Numerical accuracy | 15% | 7.5 | Correct units for all three rate constants (k₂: dm³ mol⁻¹ s⁻¹; zeroth: mol dm⁻³ s⁻¹; 5/2: dm^7.5 mol^-2.5 s⁻¹) with dimensional analysis shown; proper handling of fractional order units | Correct units for k₂ and zeroth order but error in fractional order; or correct final answers without derivation | Incorrect units for any rate constant; fundamental misunderstanding of order-unit relationship |
| Diagram / structure | 20% | 10 | Clear T/R hemoglobin diagrams showing Fe displacement and subunit arrangement; well-drawn cis and trans stereoisomers with proper 3D representation and mirror images for optical activity; labeled heme structures for cytochromes b and c | Basic diagrams with some structural features; stereoisomers drawn but lacking clarity in optical activity demonstration; minimal or unclear heme structures | Absent or incorrect diagrams; confused stereoisomer representations; no structural illustrations for heme proteins |
| Application context | 15% | 7.5 | Connects hemoglobin cooperativity to physiological O₂ transport efficiency and 2,3-BPG regulation; relates cytochrome c oxidase to mitochondrial disease and Indian research on Complex IV deficiency; links pi-bonding to organometallic catalysis relevance | Mentions biological significance without specific examples; generic statement on enzyme importance; limited connection to real-world applications | No application context provided; purely theoretical treatment without biological or practical relevance |
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